Condensing furnaces have been used to improve heating efficiency from conventional mid-efficiency furnaces for a number of years in residential heating applications. Typical conventional gas furnaces employ a burner to combust a gaseous fuel, a primary heat exchanger for transferring heat from combustion gases to the circulating air stream, a blower to circulate return air from the space to be heated over the external surfaces of the heat exchanger and through a duct system, providing warm air to the home or structure. These furnaces often include an induced draft fan to draw out and vent the flue products from within the primary heat exchanger. Condensing furnaces typically employ a secondary (recuperative) heat exchange section to transfer additional heat from the combustion products after they have passed through the primary heat exchanger thereby improving the operating efficiency of the furnace.
The main differences between a conventional furnace and a condensing furnace are the heat exchanger technology used to extract heat from the combustion process and the method used to exhaust the combustion gases. Conventional furnaces, such as mid-efficiency conventional furnaces, are designed to transfer sensible heat from the combustion gases to the heat exchanger as they are cooled. However, combustion gases may remain above their dew point temperature, and exit the heat exchanger into the exhaust vent at temperatures typically in excess of 250° F. (121° C.). In these embodiments, heat is lost through the vent and limits thermal efficiency such that, in some cases, maximum thermal efficiency is limited to about 83%. In contrast, condensing furnaces are designed to transfer sensible heat and also transfer latent heat during condensation. These features may provide a higher rate of heat transfer to the circulating supply air of a heating system than conventional furnaces. Combustion gases are cooled below their dew point temperature in the recuperative heat exchanger and exit the heat exchanger into the exhaust vent at temperatures typically below 130° F. (54° C.). Therefore, in some cases, condensing furnaces may achieve maximum thermal efficiencies that are greater than 90%.
Condensing furnaces may provide a longer dwell time of heated flue gases than conventional furnaces, even to the point where the combustion exhaust gases “cool” and condense. This longer dwell time has been accomplished by using two heat exchangers, one for primary heat exchange and one for secondary heat exchange. The primary heat exchanger communicates with the secondary recuperative coil through a common flue gas collector box. Circulating air flow across the heat exchangers may be directional and limited by furnace or heat exchanger design.
In the primary heat exchanger, sensible heat is transferred from the combustion products as they are cooled while the temperature of the combustion gases remain above their dew point. Generally, primary heat exchangers may include a plurality of tubes, formed sections, or a single cavity “drum” to house the combustion of the gaseous fuels therein. This primary or leading heat exchanger assembly section may be made from conventional heat exchanger materials (i.e., aluminized steel, 409 SS, 304 SS). The secondary (recuperative) heat exchanger assembly may be made with a plurality of tubes using high grade corrosion resistant materials and is used to transfer additional heat from the combustion gases. This secondary heat exchanger cools combustion products sufficiently to condense a portion of the water vapor in the combustion products. Furnaces can achieve efficiencies exceeding 90% by condensing a portion of the water vapor produced as a standard by-product of the combustion process present in the flue gases, utilizing the latent heat of vaporization (972 Btu/lb. of water condensed).
Residential high efficiency furnaces may be typically located in a heated space (non-weatherized) and the air to be heated is return air from the heated space. In these furnace designs, air typically traverses the heat exchanger sections in series, i.e. over the secondary heat exchanger first and then over the primary heat exchanger assembly. The circulating air stream passing over the heat exchanger sections is typically within 5 to 10° F. (1 to 3° C.) of the conditioned space temperature.
Commercial heating furnaces and duct furnaces are often located outdoors (weatherized), typically on rooftops and therefore are exposed to outdoor temperatures. Commercial heating products are also used to provide ventilation air from outdoors (make-up air) to meet ASHRAE and EPA indoor air quality requirements. This air must then be heated to maintain comfort temperatures in the ventilated area. Even in make-up air applications where the heating apparatus is located indoors, the heating apparatus may be exposed to different conditions, for example, 100% outdoor air (circulating air) may be directed across the heat exchanger and, therefore, may initially traverse the heating unit at outdoor temperatures. In extreme northern climates, entering air may be below 0° F. (−17.8° C.). This can result in reduced heat exchanger surface temperatures and reduced temperatures of the combustion gases inside the heat exchanger.
Currently, “non-weatherized” (indoor installations) condensing furnace designs direct entering air flow over the secondary heat exchanger first, which is suitable for entering air temperatures that are above 40° F. (4.4° C.). Typically, manufacturers of this type of condensing furnace include warnings about limiting the minimum inlet supply air temperature to 40° F. (4.4° C.) and locating the furnace in a space where the temperature never drops below 40° F. (54° C.). Otherwise, these conditions could result in freezing of condensate in the condensing recuperative coil assembly and preclude their use in “weatherized” (outdoor) applications.
Current and proposed building efficiency standards (ASHRAE, DOE and Canadian Provincial authorities) are mandating higher efficiencies, particularly for weatherized commercial furnace applications (i.e., rooftop or make-up air type furnaces). Clean Air and Green Building initiatives are also requiring increased ventilation rates, necessitating higher heat inputs to temper the make-up air.
For commercial applications, one design variation of a conventional high capacity mid-efficiency weatherized furnace is a drum and tube design. This design typically provides two (2) or three (3) heat exchanger passes to transfer sensible heat from the combustion gases. The first pass incorporates a single larger diameter tube or drum into which the gas burner fires. Typically this burner is a forced draft type. The second pass (and third pass if used) employ multiple tubes to transfer sensible heat to the circulating air stream. The passes are disposed in series to the circulating air stream, i.e. the supply air sequentially passes over the first pass tube or drum, then the second pass tubes and then the third pass tubes (if provided). Combustion gases exiting the last pass of the heat exchanger into the exhaust vent are in excess of 250° F. (121° C.). Efficiencies for this design are between 80 and 83%. Commercial heating systems may also operate with varying gas firing rates or gas input ratings. “Turndown” is a ratio that refers to the operational range of a furnace and may be defined as the ratio of the maximum heat output to the minimum level of heat output at which the heat exchanger may operate efficiently or controllably. Modulating furnaces provide improved annual fuel utilization efficiencies by maintaining nearly constant temperatures in the heated space by varying heat input based on measured temperatures of the supply (outdoor) air. Applications resembling variable air volume (VAV) and zoning systems allow air pressures in the building to remain stable by varying the supply air or directing air into different zones. Higher turndown is beneficial in these applications because the heat input can be matched with the varying supply of airflow to maintain the desired space temperatures and building pressures while operating within the furnace manufacturer's specifications.
In these condensing heat exchanger assemblies, latent heat is extracted from the products of combustion and condensation of water vapor occurs. Additionally, this condensate contains hydrochloric and sulfuric acids resulting from the high temperature combustion process. The condensate produced is therefore acidic, typically with a pH in the range of 4.0-6.0, which is mildly corrosive to conventional heat exchanger materials.
The primary corrosion mechanism with condensing furnaces is wet-dry cycling. Areas where condensate forms and then dries out tend to concentrate the acids contained in the condensate leading to corrosion and possible failure of even the most corrosion resistant materials used in heat exchanger construction. This is most likely to occur in the primary heat exchanger where the largest temperature differential occurs during the heating cycle.
There have been issues with condensing furnaces related to the risk of condensate freezing especially in outdoor “weatherized” and make-up air applications. Due to the acidic nature of condensate from combustion gases in condensing furnaces, a corrosion risk is present. Many conventional condensing furnaces also lack flexibility of design related to the directionality of airflow across the heat exchanger sections.